export of multiple animated objects multiplies animations by objects count
Ok so apparently it is impossible to have multiple objects in the same FBX or DAE. I've seen demos that are cheating by merging all objects into an unique armature (so it's an illusion of multiple objects).

The solution I've taken is to have a root bone for each object at 0 0 0 (independant of the next ones) then a bone that's offseting object at right place in the scene. then copy-paste each armature/object in a new blend file. And a checking file that links all scenes of each object (I'm not enthusiast to merge all in a single gigantic armature, because animation in/out/loops are forced to be at same length, it gives no possiblity of independant looping, and no possiblity of re-use independant objects / animations).

It's the best solution I've found. If you have a better workflow for working with multiple animated objects that needs to be synced in a precise timing (like an object jumping onto another when the second one has finished it's apparition), that needs to be exported/readable/triggerable in FBX or DAE, don't hesitate to talk about it.